The pelvis is a complex made of three bones: the sacrum and the left and right os coxae, also called "innominate" bones. The sacrum forms the posterior part of the pelvis, and is made up of fused vertebrae. Many primates have a tail extending from the end of the sacrum; humans and apes have only a small number of tiny vertebral bodies, called the coccyx, or ``tailbone.''
The innominate bones (os coxae) make up the sides and the front of the pelvis. Each innominate bone is itself composed of three fused bones:
- The largest part of the innominate bone, this forms the upper blade, which flares outward to make a bowl-shaped cavity supporting the abdominal organs.
- The ischium is the most inferior part of the pelvis, the part that most primates sit on.
- The pubis is in the front of the pelvis. The two pubes meet at the midline at the pubic symphysis.
The three bones meet in the center of the socket for the hip joint, called the acetabulum. The bones fuse together during childhood, so that adults do not have any marking showing the boundaries between them.
A few other features of the innominate bones give important information about sex or locomotion.
What to do: Take some time to orient yourself on the pelves at this station. Be sure to be able to identify the acetabulum, pubic symphysis, and the sacrum, ilium, ischium and pubis.